Problem sending emails to bt addresses

I have a 'btinternet' address and for 4 days all messages sent to btinternet or btopenworld addresses are bouncing back with MAILER-DAEMON - failure notices. Says: remote host said 554 message not allowed. Sending OK to other addresses and some messages to me from btinternet address are being bounced back to sender. Any ideas please? Have made no recent changes to computer.